1. Field of the Invention
This invention relates to an improvement in a distance measuring device of the active type arranged in camera and the like, etc.
2. Description of the Related Art
In this kind of device, there have been many previous proposals for measuring a distance to an object on the basis of the principle of triangulation by projecting a light signal onto the object and finding a position at which the light signal reflected from the object is received and which varies depending on the distance to the objects. In these proposals, generally, a light receiving element such as a PSD having two outputs of which the ratio varies according to the receiving position of the reflected light signal is used. By detecting the ratio of the two outputs, the receiving position for the reflected light signal can be detected. To seek the ratio of the two outputs, for example, the two outputs are logarithmically compressed and, then, are subtracted one from the other. Of the systems employing this method, there is the dual slope integration type as disclosed in Japanese Laid-Open Patent Application No. Sho 60-19116 or others. This system has merits such that only one integration circuit suffices contributing to a small circuit scale and that the problem of matching the circuit constants is not encountered since one circuit is used in common to carry out both integrations as they take place time-divisionally. The distance measuring technique utilizing the dual slope integration is briefly explained below.
Referring to FIG. 8, while a light signal is projected onto an object whose distance is to be measured, a position at which the light signal reflected from the object is received is detected by a semiconductor position detector 51 (hereinafter referred to as PSD) having electrodes 51a and 51b at either end. Reference numeral 52 represents a spot light which is formed by the reflection of the light signal projected onto the object. The PSD 51 outputs photocurrents indicative of the position of the center of gravity of the spot light 52 from the respective electrodes 51a and 51b. Now, letting the length of the space between the electrodes 51a and 51b of the PSD 51 be denoted by L, the distance from the electrode 51b to the center of gravity of the spot light 52 by x, and the photocurrents output from the electrodes 51a and 51b by Ia and Ib, respectively, the following equation is obtained: EQU Ia/(Ia+Ib)=x/L . . . (1)
Thus, Ia/(Ia+Ib) is in proportional relation with the relative position x of the center of gravity of the spot light 52. To find the value of Ia/(Ia+Ib) is to find the center of gravity of the spot light 52. Since, as has been described above, the position of the spot light 52 corresponds to the distance to an object to be photographed, the value of Ia/(Ia+Ib) takes a value corresponding to the object distance.
FIG. 9 shows an integrator for performing computation of the equation (1) comprising an integration capacitor 53, an amplifier 54 constituting a Miller integration circuit together with the integration capacitor 53 and a switch 55 for selecting either one of the two outputs of a circuit (not shown) which represent the photocurrent Ia and the reversed photocurrent Ia+Ib. Now, suppose with the integration capacitor 53 having no charge at all, the switch 55 is set in its "a" position to integrate the photocurrent Ia for a period of time T (hereinafter called a "first integration"). Then, the switch 55 is moved to and, set in, its "b" position to integrate the photocurrent (Ia+Ib) in the reverse direction (hereinafter called a "second integration"), wherein it takes a period of time "ts" to return the charge on the integration capacitor 53 to zero. During this process, the output V.sub.0 of the amplifier 54 varies as shown in FIG. 10. Here, when the integration capacitor 53 has a capacitance C, as is apparent from FIG. 10, the following equation is established: EQU Vos=Ia.multidot.T/C=(Ia+Ib)ts/C . . . (2)
By rearranging this, EQU ts=(Ia/(Ia+Ib).multidot.T
is obtained.
Since the Ia/(Ia+Ib) corresponds to the position x of the center of gravity of the spot light 52 (see the equation (1)), the following equation is obtained by the equations (1) and (3): EQU ts=(x/L).multidot.T . . . (4)
Thus, the second integration time "ts" has a value corresponding to the position x of the spot light, hence, to the distance to the object.
It should be noted that although the foregoing discussion has been made as if the photocurrents output from the PSD 51 are directly integrated, in actual practice the first integration is preceded by amplification with a number of successive amplifiers. It should also be noted that a light projecting means, for example, an infrared light emitting diode (hereinafter referred to as iRED) is made to be modulated at a frequency of several KHz to several tens of KHz in order to discriminate it from the ambient light. Meanwhile, even in the integrator, it is the general practice that the integration is carried out in synchronism with the frequency of modulation of the IRED.
By the way, the system employing above-described dual slope integration has the following problems.
Firstly, because the integration is time-divisionally performed, the magnitude of the signal, i.e. the strength of the reflected light from the object, in the first integration time must be equal to that of the signal (the strength of the reflected light) in the second integration time, otherwise an error of distance measurement would be produced. In other words, the signal must be constant during the distance measurement. In a case where a projected light spot moves on the object across its parts of different reflectance due to, for example, camera shake, etc., an error of distance measurement would be produced. If the integration time is shortened, this problem can be improved. However, conversely, a deterioration of the S/N ratio will be invited. This is because the use of the synchronous integration makes the ratio of the mu-factor of the frequency component of the signal to the mu-factor of the frequency component of the other, i.e. the S/N ratio, better in proportion to the cubic root of the integration time.
Another problem that becomes serious is that the efficiency of light emission lowers as the temperature of the iRED increases during the energization. This is because the flow of electric current through the iRED leads to an increase in its temperature which in turn causes the efficiency of heat generation to lower. Therefore, for even the same current value, the power of light emission after a certain time from the start of energization is different from that at the start time. Hence the signal itself is caused to be ever changing during the integration. This condition is shown in FIG. 11.